Domain name system problems left some users of Sophos unable to get security updates on Friday. The same issue, blamed on a mistake by one of the security firm's service providers rather than hostile action, left many surfers unable to access its main sophos.com website.
Graham Cluley, senior technology consultant at Sophos, …

@Situation Normal then

Sophos-DNS isn't hosted externally

It's somewhat nice reading that they blame an external provider for their DNS-issues - according to WHOIS they run their own DNS-servers, and a tracert to one of them proves this:

Tracing route to ns4.sophos.com [213.31.172.25]

over a maximum of 30 hops:

1 <1 ms <1 ms <1 ms BLANKED OUT

2 1 ms <1 ms <1 ms BLANKED OUT

3 8 ms 8 ms 7 ms BLANKED OUT

4 8 ms 7 ms 8 ms BLANKED OUT

5 11 ms 11 ms 10 ms BLANKED OUT

6 11 ms 11 ms 11 ms BLANKED OUT

7 12 ms 12 ms 12 ms BLANKED OUT

8 12 ms 12 ms 12 ms t2c2-ge12-0-0.de-fra.eu.bt.net [166.49.172.101]

9 12 ms 11 ms 12 ms t2a4-prc2.de-fra.eu.bt.net [166.49.172.52]

10 19 ms 18 ms 19 ms frankfurt55.de.eqip.net [166.49.147.174]

11 27 ms 26 ms 26 ms 195.90.66.74

12 27 ms 28 ms 27 ms 195.206.65.37

13 30 ms 31 ms 30 ms sophos-abingdon.gb.eqip.net [213.31.192.130]

14 32 ms 32 ms 32 ms ns4.sophos.com [213.31.172.25]

15 32 ms 31 ms 33 ms ns4.sophos.com [213.31.172.25]

Trace complete.

I assume that the second nameserver is also being maintained by Sophos, directly, but due to requirements of DNS-servers having to sit in different IP-address-segements this one doesn't resolve directly into Sophos HQ, naturally.